Sometimes, you need to use the same text in more than one place. For example, you may want to use fragments of your software manuals in your software's site.
If you are part of the Python-verse then it's likely that your docs are written using Sphinx. Ǹikola and Sphinx share roots in reStructuredText so they are already compatible in many important ways.
However, Sphinx adds a whole bunch of extensions to reStructuredText and Sphinx users use them. So, what's a dev to do? He'll write code. So I wrote a nikola plugin which adds support for a large chunk of the Sphinx-specific markup.
How big a chunk? Quite big since it adds support for over 35 extensions, roles and directives (no, toctree is not one of them yet).
This way, you may be able to enjoy a good documentation-specific tool and a (if I may say so myself) good website tool from common sources.